Calcitriol as the active form of Vitamin D is actually a hormone over
1,000 times as potent as natural or synthetic vitamin D itself. It is
normally created in your pet's body after plants and/or animal tissues are
eaten. The ingredients to make the calcitriol must first make their way
through the liver and then migrate to the kidneys which if healthy do the
final step of making the calcitriol. Calcitriol helps your pet's body absorb
calcium and phosphate from food and put it in growing bones, where extra calcium
and phosphate is stored. It is vital to your pet's health that an appropriate
amount of calcium and phosphates are available in the blood, and absorption
from food or resorption from bone are controlled by calcitriol for this most
critical control of blood calcium. A molecule called parathyroid
hormone (PTH) works with calcitriol at bone to release calcium to blood. Its
gland of origin is the main detector of low blood calcium, and the PTH
secreted stimulates formation of more calcitriol in the kidney. The
calcitriol once normalized inhibits further formation and secretion of PTH
(called feedback inhibition). Parathyroid hormone also recaptures calcium
otherwise lost to urine so it has many benefits when in normal levels. When
too high, however, PTH becomes a serious health problem and we wish to lower
it which we can do by giving calcitriol.

If your pet's kidneys are failing and no longer doing their job of making
enough calcitriol the parathyroid glands (around the thyroid gland in the
throat area) produce more PTH in an attempt to normalize calcium
and phosphate in the blood. Although we would prefer to not let PTH increase
at all in our patients with kidney disease, the PTH is not too damaging until
it reaches about 3 times the upper limit of its normal level in the
blood as this is when toxicity due to the PTH first appears. This is usually
happening about when your pet first appears to be ill. The PTH increase
is worsened by the failure of the sick kidneys to eliminate phosphorus from
blood which then gets too high a level. This both slows calcitriol
formation and also prevents calcitriol from working at the parathyroid gland
to block the PTH excess. When at very high levels late in kidney failure the
phosphorus combines with blood calcium to precipitate into soft tissues
damaging them. Although very injurious, this only happens at late
stages of disease and use of calcitriol should prevent getting to such stages
nearly as soon as would happen in pets not given calcitriol. The high levels of
PTH directly damage kidney cells long before the mineral precipitation phase
of renal injury.

How prescribed calcitriol works:

By taking a carefully measured dose of calcitriol in capsule or liquid
form we intend to keep the parathyroid glands from producing a
toxic level of PTH; the amount of calcitriol prescribed needs to be
calculated specifically for your cat or dog by your veterinarian and can be
prepared for you by a special compounding pharmacy. The prescription of
calcitriol for your pet is designed to be enough quantity to shut off the PTH
production but not so much as to create elevated levels of calcium and
phosphate in the blood. Because PTH levels are difficult to control
once kidney failure is well underway, it is desirable to administer
calcitriol at low doses as early as possible in kidney failure to avoid PTH
excess from occurring in the first place.

Nearly 2000 pet owners and 250 veterinarians who used calcitriol were
surveyed recently. Over 80% of the owners reported that their pets were
brighter and more social and had better appetites when taking calcitriol. It
was also felt that these animals had a substantially longer life span than
patients which were not receiving calcitriol. A study of cats showed
that 29 cats which had PTH lowered lived over a year longer on average than
did 21 cats which did not have their PTH lowered.

Cautions:

Too high doses of calcitriol can elevate blood calcium and to a lesser extent,
phosphorus. The calcium level needs to be monitored and if it elevates, the
calcitriol prescription needs to be changed to every 3.5 days with possible
fasts preceding and following dosing [see dosing
and fasting on this web site for more details]. An intermittent dose (3.5 times
the original daily dose) is given every 3.5 days. Then, the calcium
level should be rechecked in seven days. If the calcium increase was caused
by the original calcitriol prescription, the intermittent dosing will lower
the calcium.

Due to the complex nature of hormone interactions, inadequate calcitriol
can also cause calcium elevation. Your veterinarian can direct you further to
determine the most appropriate calcitriol dosing strategy. Most moderate
increases of total calcium seen in patients with damaged kidney are caused by
factors not related to calcitriol. They are not clinically important so
only if ionized calcium is increased is there need for any concern.

If your pet has a plasma phosphorus level greater than 6 mg/dl, calcitriol
might not succeed in lowering the toxic levels of PTH. In this case, the
plasma phosphorus must be reduced by diet, fluid administration or by
phosphate binders ~ calcitriol can then be prescribed. The above-described
every 3.5 days intermittent dosing approach also works to reduce phosphorus
to the small extent that phosphorus elevation might have been caused in part
by previous calcitriol doses.

It is helpful to have your pet's PTH level monitored by serum assays when
it is on calcitriol therapy in order to determine whether any adjustment to
the calcitriol dosage should be made. This monitoring is not necessary when
calcitriol is used early in a preventative mode, however .